Attribute 'sn' not allowed help

Hi,
We have this wierd problem.Seems like everyhitng is right but can't get over
this.we have included all requried shemas.
Each time we tried to all entry wiht 'sn' in it spits out this error.
objectclass voilation:Attribute 'sn' not allowed
Please need some light here.
Thanks for your time,
Partha